In the manufacture of motor vehicles, windshields and other fixed-panel windows are attached to the vehicle body by means of a polymeric sealant. In order to obtain maximum bond strength between the glass window panels and the coated vehicle body, some surface preparation of the vehicle body coating is typically required. Because the bead of window sealant material is generally applied to a vehicle body which has already been painted or otherwise coated, it is usually necessary to use a primer material to prepare the surface to receive the sealant. Once the sealant bead is applied to the coated vehicle body, the glass panel is then installed onto the vehicle so that the bead of sealant comes into firm contact with both the painted substrate surface and the glass panel. After the sealant bead has properly cured, the vehicle must pass government Motor Vehicle Safety Standards (MVSS) tests which include the crash-worthiness of the windhsield installation.
The Safety Standards provide no margin for failure of the windshield bonding system. In crash and roll-over situations, the windshield provides additional support of the vehicle roof and must remain bonded to the vehicle body.
Motor vehicles today are coated with a wide variety of paint types and coating systems. Each paint type has its unique formulation and surface characteristics, once dried. Even minor changes in the formulation of a paint may lead to adhesive failure of the bond between the window sealant material and the painted vehicle body. Typically, in the automotive industry there are frequent changes in the formulations of paints which are employed. Some of these changes are motivated by aesthetic considerations, others by more practical concerns. Changes in the paint formulation may relate to modification of solvent types or reduction in solvent content of the formulation, changes in the nature and concentration of paint additives, yearly color styling changes, and other changes dictated by the user's location and paint application requirements.
Typical coatings systems used in the automotive industry today involve the application to the vehicle body of 1) an electrocoat paint coating, 2) a primer-surfacer coating, and 3) a decorative paint coating. In some cases, notably truck bodies, the decorative coating takes the form of a pigmented topcoat. In the case of automobiles, the industry is tending toward increased use of a decorative coating made up of a thin pigmented basecoat layer which is overcoated by one or more thicker, clear, topcoat layers. So-called "metallic" color styles, which contain flakes of mica, aluminum, or other metal in the pigmented basecoat, are becoming increasingly popular.
In a typical "metallic" basecoat/clearcoat coating system, this means that there may be as many as seven interlayer interfaces between an applied bead of sealant and the underlying metal vehicle body: 1) the sealant to sealant-primer interface; 2) the sealant-primer to clear topcoat interface; 3) the clear topcoat to pigmented basecoat interface; 4) the basecoat to metallic flake interface; 5) the basecoat to primer-surfacer interface; 6) the primer-surfacer to electrocoat interface; and 7) the electrocoat to vehicle body interface. The potential exists for delamination or adhesive failure at one or more of these interfaces when stress is applied. In a practical sense, however, delamination of the electrocoat from the underlying steel vehicle body is a virtual impossibility because of the nature of the electrochemical process by which that paint layer is applied.
There is thus a need in the art for a sealant composition and sealing system which provides a direct firm, durable, weather-resistant seal between motor vehicle fixed-window glass panels and the electrocoat paint layer of the motor vehicle body.